This invention relates in general to vehicular transmissions and in particular to a combined neutral sensing and shift lever interlock assembly that is adapted to sense when all of the shift rails of the transmission are in a neutral gear ratio and to prevent more than one shift rail from being shifted out of a neutral position at any given time.
In virtually all land vehicles in use today, a transmission is provided in a drive train between a source of rotational power, such as an internal combustion or diesel engine, and the driven axle and wheels of the vehicle. A typical transmission includes a case containing an input shaft, an output shaft and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
Many transmission structures are known in the art for performing these gear ratio selections manually, i.e., in response to some physical exertion by the driver of the vehicle. In a conventional manual transmission, the driver grasps and moves an upper portion of a pivotable shift lever to effect shifting of the transmission. In response thereto, a lower portion of the shift lever engages and moves one of a plurality of shift rails provided within the transmission. The shift rails are typically supported within the transmission case for sliding movement from a central neutral position either axially forwardly to a first gear engaging position or axially rearwardly to a second gear engaging position. Thus, the selection and movement of a particular shift rail causes certain ones of the meshing gears to be connected between the input shaft and the output shaft. As a result, a desired gear ratio is provided between the input shaft and the output shaft. Manually operated transmissions of this general type are well known in the art and are relatively simple, inexpensive, and lightweight in structure and operation. Because of this, the majority of medium and heavy duty truck transmissions in common use today are manually operated.
As is well known, when the engine of the vehicle is initially started, it is desirable that all of the shift rails of the transmission be positioned in the central neutral position. If any of the shift rails of the transmission are positioned outside of the central neutral position in a gear engaging position when the engine is started, the vehicle may experience sudden and unintentional movement. To prevent this undesirable situation from occurring, most manually shifted transmissions are provided with a neutral sensing structure that is designed to disable the starter system of the vehicle when any of the shift rails of the transmission are positioned out of the central neutral position. A typical neutral sensing structure includes a sensor that is responsive to the position of the shift rails and a neutral relay that is connected in the starter circuit of the vehicle. When all of the shift rails are positioned in the central neutral position, the sensor causes the neutral relay to be closed, thereby enabling the starter circuit of the vehicle to start the vehicle engine. When any one of the shift rails is positioned out of the central neutral position, the sensor causes the neutral relay to be opened, thereby disabling the starter circuit of the vehicle and preventing the engine from being started.
As is also well known, after the vehicle engine has been started, it is desirable to affirmatively prevent more than a single one of the shift rails from being located out the central neutral position in order to prevent damage from occurring to the transmission during use. To accomplish this, it is known to provide the transmission with an interlock that is responsive to movement of one of the shift rails out of the neutral position for positively maintaining all of the other shift rails in the neutral position. A typical interlock includes an upper body portion that is engaged by the lower end of the shift lever and a pair of depending arm portions that selectively engage the shift rails. The interlock is supported for lateral movement with the shift lever when the shift lever is initially moved to select a particular one of the plurality of shift rails for subsequent movement. However, the interlock is prevented from axial movement with the shift lever when the shift lever subsequently moved to shift the selected shift rail to a gear engaging position. Thus, the non-selected shift rails are positively maintained in the neutral position by the interlock when the selected shift rail is moved to a gear engaging position.
A variety of neutral sensing structures are known in the art and have functioned satisfactorily. However, known neutral sensing structures are relatively complex and expensive in structure and cannot be retrofitted to existing transmissions without disassembly thereof. Thus, it would be desirable to provide an improved structure for a neutral sensing structure for a vehicular transmission that is relatively simple and inexpensive in structure and can be retrofitted to existing transmissions without disassembly thereof